1. Field of the Invention
The present invention relates to an electronic component having at least two electrode pads on a surface thereof, an assembly comprising such electronic component and metallic soldering elements usually called "solder bumps", and an electronic component unit comprising such electronic component and another electronic component.
2. Description of the Prior Art
In the conventional method of bonding a semiconductor device to a substrate with solder bumps disposed therebetween, as disclosed in U.S. Pat. No. 5,216,278, the bumps are formed on the semiconductor device at predetermined intervals, electrode pads to be soldered are of the same area, and balls for forming the bumps are of the same size. In the conventional method, a semiconductor device having such a junction structure is bonded to a circuit substrate regardless of warpage of the substrate on which electrode pads are also formed, and also regardless of expansion characteristics of the substrate.
FIGS. 8 and 9 show an example of electronic component units comprising a semiconductor package and a circuit substrate which are connected by the above conventional method,
In these drawings, a semiconductor element 1 is mounted on a base 3 and electrically connected to top electrodes 5a of the base 3 by wires 4. The top electrodes 5a of the base 3 are respectively electrically connected to bottom electrodes 5b. The semiconductor element 1, the wires 4 and the base 3 are sealed with a resin 5. Insulating resist 2 is coated on the lower side of the base 3. The surfaces of the lower surface electrodes 5b of the base 3 have portions without the resist 2 which respectively form electrode pads 6. Solder balls 8 are respectively adhered to the pads 6.
On the other hand, resist 2a is coated on the surface of a substrate 9, and the surfaces of electrodes 10 in the substrate 9 have portions without the resist 2a, which respectively form electrode pads 7. The semiconductor package is mounted on the substrate 9 by melting the solder balls 8 by heat and bonding the balls to the pads 7.
A structure electrically connecting a semiconductor package to a circuit substrate via solder bumps exhibits excellent high-speed electrical processing because the wiring length is decreased by an amount corresponding to the lead length and, thus, conforms to a multi-pin structure and the use of multiple pins because many bumps can be formed, as compared with the connection to a circuit substrate through leads. In this bump connection structure, the bump arrangement area is decreased and the packaging density of the package is increased as the diameter of each solder bump is decreased. The suitable size of each bump, therefore, is considered to be 500 to 700 .mu.m. However, a decrease in size of the bumps causes an increase in cost due to the need for advanced bonding technology, and cause a significant decrease in the strength reliability of the bumps. The problem with respect to the reliability is considered particularly significant partly because the difference in thermal expansion between a substrate on which a chip is mounted and a circuit substrate is accommodated by the solder bumps having a low strength. This is also because warpage invariably remains in the substrate, and the warped substrate is bonded to the circuit substrate, so that tensile loads are apt to be applied to the bumps.